Process for controlling the emission of flue gases

ABSTRACT

Provided is a process for controlling heavy metal (e.g., mercury) emission in S0 2 -containing flue gas, comprising passing a stream of the flue gas ( 2 ) through a wet scrubber ( 1 ) where it is brought into contact with a liquid absorbent ( 8 ) comprising an ionic liquid, an oxidizer ( 11 ) and polar protic organic solvent, wherein the amount of said organic solvent is adjusted such that the SO 2  absorption is minimized while operating said wet scrubber at a temperature lower than the normal working temperature that would be used in the absence of said solvent. The process may be preceded by an initial stage where a fluoride-containing liquid is used for reducing the amount of sulfur dioxide in the flue gas stream; the sulfur dioxide can be subsequently desorbed from the fluoride-containing liquid upon the addition of a polar solvent.

Flue gases formed by the combustion of fossil fuels, for example, inelectric power generating plants, need to be treated in order to removepollutants and toxic substances prior to the release of the gases intothe atmosphere.

Elemental mercury volatilizes upon combustion of coal, becoming acomponent of the flue gas. Elemental mercury is considered asenvironmentally hazardous pollutant. Some processes for decreasingmercury emissions into the atmosphere involve the addition ofhalogen-containing sorbents directly to the coal prior to combustion,and/or following the combustion, i.e., to the flue gas.

Recently, the use of halide-containing ionic liquids combined withoxidizers such as iodine was disclosed for absorbing and oxidizingmercury. US 2011/0081286 describes an apparatus in the form of a wetscrubber, through which the flue gas is caused to flow upward and tocontact the ionic liquid and the oxidizer. The treated gas stream whichexits the scrubber exhibits decreased mercury emission. The temperatureof the scrubber may be up to 200° C., e.g., between 40 and 80° C., andliquids such as water, alcohol, ethylene glycol, polyethylene glycol,DMSO, DMF and methylnaphthalene can be added to the ionic liquid in thescrubber.

Sulfur dioxide present in the flue gas is also absorbed by ionicliquids, interfering with the removal of mercury. This is because sulfurdioxide reduces the oxidizer associated with the ionic liquid throughthe following chemical reaction:

SO₂+Ox+2H₂O→H₂SO₄+2H⁺+Ox²⁻  [I]

wherein “Ox” denotes the oxidizer. For example, when the oxidizerassociated with the ionic liquid is elemental iodine, the reductionreaction shown above results in the conversion of iodine to iodide, suchthat the oxidizer (elemental iodine) is no longer available foroxidizing the mercury.

In view of the above, there exists a need to minimize the absorption ofsulfur dioxide in the ionic liquid used as a medium for oxidation ofmercury, ensuring effective conditions which would allow the removal ofmercury from the flue gas. The present invention addresses this need bymeans of a process involving either the modification of the propertiesof the ionic liquid, in order to reduce its affinity towards sulfurdioxide, such that mercury can undergo oxidation even when sulfurdioxide is present in the flue gas, or by the separation of sulfurdioxide, which is also an environmental pollutant, from the flue gasbefore it is subjected to the step of mercury removal, or both.

In this regard it should be noted that the removal of sulfur dioxide isconventionally accomplished through the use of the wet flue gasdesufurization (WFGD) process, in which the flue gas flows in an upwarddirection through a suitable tower (a gas-liquid contactor) and contactstherein with calcium-containing slurry (e.g., limestone). The sulfurdioxide is absorbed in the slurry and is subsequently allowed to reactwith the calcium compound in a suitable reaction vessel. The by-productthus formed is gypsum (CaSO₄). Recently, ionic liquids have beensuggested for use as absorbents in connection with sulfur dioxide. Forexample, US 2010/0015040 disclose a method for separating and recyclingsulfur dioxide from a gaseous mixture by using ionic liquids. In thismethod, the absorption and separation of the SO₂ is performed at 20-50°C., and the stripping of the SO₂ is performed at 120-250° C. A similarmethod was described by K. Y. Lee et al. [Int. J. of Hydrogen Energy,33, p. 6031-6036, (2008)]. The same group also investigated ionicliquids with bromide, chloride and iodide anions and their influence onSO₂ solubility [Bull. Korean Chem. Soc. vol. 31, No. 7, pages 1937-1940(2010)]. Prasad et al. [J. Phys. Chem B. 113 4739-4743 (2009)] alsoinvestigated the solubility of flue gas components (e.g., SO₂ and CO₂)in ionic liquids. Furthermore, U.S. Pat. No. 4,973,456 and U.S. Pat. No.5,338,521 describe the removal of acid gases from gaseous mixtures usingfluoride salts. In U.S. Pat. No. 5,338,521, the use of molten salts isdescribed, which capture the acid gases and subsequently, uponsolidification, release the acid gases.

As noted above, one embodiment of the invention is a process whichallows the removal of mercury from a flue gas by means of absorption andoxidation in an ionic liquid, even when the “competitor” sulfur dioxideis present in the flue gas to be treated.

It should be noted that the affinity of the ionic liquid toward sulfurdioxide is temperature dependent, whereas its affinity toward mercurydoes not depend on the temperature. As the working temperature of thewet scrubber increases, the affinity of the ionic liquid toward SO₂molecules decreases, namely, high working temperature favors mercuryabsorption over SO₂ absorption.

It has been found that it is possible to diminish the absorption ofsulfur dioxide by the ionic liquid through the combined effect of theworking temperature, the use of an ionic liquid with an anion that isrelatively large and not highly electronegative, and the addition of oneor more polar protic organic solvents to the ionic liquid.

The combination of the conditions set out above results in modifying theabsorption profile of the ionic liquid, shifting the selectivity of theabsorption process by the ionic liquid toward the mercury, rather thanits competitor, sulfur dioxide. In particular, it is noted that thepolar protic organic solvents sharply decrease the affinity of the ionicliquid toward SO₂, without having a similar effect on the affinitytoward the mercury, allowing the wet scrubber to operate at atemperature lower than the working temperature that would be used inorder to permit the selective removal of the metal in the absence ofsaid additive.

The invention therefore provides a process for controlling metalemission in SO₂-containing flue gas, comprising passing a stream of theflue gas through a wet scrubber where it is brought into contact with aliquid absorbent comprising a ionic liquid, an oxidizer and a polarprotic organic solvent, wherein the amount of said organic solvent isadjusted such that the SO₂ absorption is minimized while operating saidwet scrubber at a working temperature lower than the normal workingtemperature that would be used in the absence of said solvent.

In the presence of the polar protic organic solvent, the workingtemperature of the scrubber can be lowered by at least ten degrees. Theexact working temperature can be adjusted by the operator depending onseveral factors, including, inter alia, the ionic liquid used, theorganic solvent additive and also the composition of the flue gas, e.g.,the presence of water vapors in the flue gas. For example, when theionic liquid used is 1-butyl-3-methylimidazolium bromide and the fluegas to be treated contains water vapors, then the wet scrubber wouldnormally be operated at a working temperature of about 140° C. in orderto ensure effective mercury removal. The addition of a polar proticsolvent to the ionic liquid permits the operation of the wet scrubber ata lower temperature, e.g., from 100 to 120° C.

According to the process of the invention, the flue gas (in particular aflue gas formed by the combustion of coal in electric power generatingplants) is made to contact an absorption medium comprising an ionicliquid, an oxidizer capable of oxidizing heavy metals such as mercuryand a polar protic organic solvent. The contact between the flue gas andthe absorption medium is carried out in any kind of wet scrubber knownin the art, e.g., the wet scrubber described in US 2011/0081286.Suitably, the gas-liquid contact is accomplished in a verticalstructure, namely, in a tower or a column, in which an upward flow ofgases to be purified bubbles through a large volume of the liquidsorbent, or is allowed to mix with a countercurrent stream of thedescending liquid sorbent, as described in more detail below.

Heavy metals that may be removed from the gas stream by the process ofthe invention include mercury, vanadium, cadmium and lead. In apreferred embodiment of the invention, the heavy metal to be removed ismercury.

As set out above, the liquid absorbent employed in the process of theinvention is formed upon combining an ionic liquid with an oxidizer,followed by the addition of a polar protic organic solvent. These threecomponents of the liquid sorbent are now discussed in detail.

With respect to the ionic liquid used in the invention, in its mostgeneral form it is an ionic salt with a low melting point, such that itexists in the liquid state at the working temperature of the scrubber(below 200° C.). Ionic salts which are solid in their pure state at theworking temperature, but liquefies when combined with the oxidizer andthe organic solvent at the working temperature, may also be used.

Ionic salts which are suitable for use according to the invention have anitrogen-containing cation, e.g., quaternary ammonium cation, namely,NR₁R₂R₃R₄ wherein each of R₁, R₂, R₃ and R₄ is independently C1-C12alkyl group (such as methyltrioctyl ammonium). Other suitable cationsare positively charged nitrogen-containing rings such as theimidazolium, pyridinium or pyrrolidinium cations in which the nitrogenatom(s) are linked to C1-C12 alkyl groups, such as1-alkyl-3-methylimidazolium, 1-alkylpyridinium andN-methyl-N-alkylpyrrolidinium, wherein the alkyl group is preferablyC2-C12 linear alkyl group, in particular C2-05 linear alkyl group.

An inverse correlation has been observed between the size of the anionof the ionic liquid and the affinity of the ionic liquid toward sulfurdioxide. The larger the size of the anion, the lower is the ability ofthe ionic liquid to absorb SO₂ molecules. Thus, when the anion of theionic liquid is halide, the order of SO₂ absorption would be Cl⁻>Br⁻>I⁻.In a preferred embodiment of the invention, the radius of the anion ofthe ionic liquid is in greater than 0.828 Angstrom, e.g., from 0.828 to1.46 Angstrom. In terms of electronegativity, anions of relatively lowelectronegativity are preferred (e.g., less than 2.9 or even less than2.8). Especially suitable anions are bromide and iodide. Ionic radiiwere reported by Biswas at al. [Int. J. Mol. Sci. 4, 379-407 (2003)].

Preferred ionic liquids for the selective removal of heavy metalsaccording to the invention are composed of a cation selected from thegroup consisting of methyltrioctyl ammonium and1-alkyl-3-methylimidazolium, (such as 1-butyl-3-methylimidazolium), andan anion selected from the group consisting of chloride, bromide andiodide. 1-butyl-3-methylimidazolium salts are abbreviated herein[BMIMX], wherein X indicates the counter ion, e.g., halide (especiallybromide or iodide).

The ionic liquids described above are commercially available and canalso be synthesized by methods known in the art. For example,methyltrioctyl ammonium is commercially available in the form of itshalide salts as Aliquat 336; the halide counter ions can be exchangedusing known methods.

The halide nitrogen-containing ionic liquids are generally prepared by areaction of the nitrogen-containing moiety with a suitable alkyl halide.Synthetic methods for making halide ionic liquids are described, forexample, by Lee at al. [Int. J. of Hydrogen Energy, 33, p. 6031-6036,(2008)] and Wang at al. [Acta Phys.-Chim. Sin., 21(5), p. 517-522(2005)].

For example, the reaction of 1-methylimidazole with alkyl halide of theformula RX wherein R denotes an alkyl chain (preferably a linear chain)consisting of n carbon atoms (e.g., n is preferably an integer between 2and 10) and X is chlorine, bromine or iodine, affords the corresponding1-alkyl-3-methylimidazolium halide salt, as shown by the followingreaction scheme:

The reactants are used in approximately equal molar amounts, or in aslight molar excess in favor of the alkyl halide. The reactants aregently refluxed at a temperature between 50 and 85° C. for about 24 to72 hours. The formation of the ionic liquid product is accompanied by anincrease at the viscosity of the reaction mixture. Upon completion ofthe reaction, the resultant ionic liquid is washed with a suitablesolvent, e.g., diethyl ether, in order to remove residual amounts ofun-reacted starting materials. In this way, an ionic liquid, which issuitable for use in the process of the invention, is obtained.

With respect to the oxidizer, it should of course be capable ofoxidizing the elemental metal to be removed. Suitable oxidizers for usein the process of the invention include, but are not limited to, iodine(I₂), fluorine (F₂), chlorine (Cl₂), bromine (Br₂).

The oxidizer is combined with the ionic liquid, following which theoxidizer may become chemically associated with the ionic liquid, e.g., acomplex is formed between the anion of the ionic liquid and the oxidizermolecule, as indicated by the UV-Vis spectrum of the ionic liquid,exhibiting a new absorption band assigned to such complex. For example,the mixing of iodine in a solid form with halide-containing ionic liquidappears to result in the formation of the complex between the iodine andeither the chloride, bromide or iodide anions of the ionic liquid.Oxidizers in a liquid or gaseous form are added to the ionic liquid inconventional manner. The liquid formed following the addition of iodineto 1-butyl-3-methylimidazolium salts is abbreviated herein [BMIMX][I₂].

The molar ratio between the oxidizer and the ionic liquid may be in therange from 0.001:1 to 3:1. It has been found that as the molar ratiooxidizer:ionic liquid increases, the sulfur dioxide absorption of themixture decreases. Accordingly, the molar ratio between the oxidizer(e.g., iodine) and the ionic liquid is preferably not less than 1:2,more preferably not less than 1:1 (e.g., not less than 3:2 or not lessthan 2:1). This ratio can be adjusted together with the absorptiontemperature in order to minimize SO₂ absorption. Thus, in anotheraspect, the invention provides a process for controlling metal emissionin SO₂-containing flue gas, comprising passing a stream of the flue gasthrough a wet scrubber where it is brought into contact with a liquidsorbent comprising a ionic liquid and an oxidizer, wherein the molarratio between the oxidizer and the ionic liquid is not less than 1:2,and wherein the absorption temperature is preferably from roomtemperature to 170° C., e.g., from 20° C. to 35° C.

It should be noted that a mixture of two or more ionic liquids which aremiscible with one another can be used, comprising a first ionic liquidhaving an anion (e.g., halide) capable of forming a complex with theoxidizer (e.g., with a halogen molecule such as iodine), and a secondionic liquid which exhibits low affinity toward sulfur dioxide, such asBF₄ ⁻ and PF₆ ⁻. The first and second ionic liquids preferably have thesame cation. The second ionic liquid functions primarily as a solvent,providing a medium with low affinity towards SO₂; it is the first ionicliquid which serves the useful purpose of complexing the oxidizer andallowing the oxidation of the mercury. According to a preferredembodiment of this variant of the invention, the first ionic liquid is1-butyl-3-methylimidazolium bromide or iodide, with oxidizer beingiodine, and the second ionic liquid is 1-butyl-3-methylimidazolium saltselected from the group consisting the BF₄ ⁻ and PF₆ ⁻ salts.

Regarding the polar protic organic solvent, it is noted that theexperimental results reported below indicate that such solvents arecapable of modifying the absorption properties of the ionic liquid, suchthat the affinity of the ionic liquid toward sulfur dioxide is reducedsignificantly, while at the same time, its high absorption capacitytoward the mercury is maintained. Suitable solvents have boiling pointsabove 140° C. and polarity of not less than about 1.6 Debye (at 25° C.),e.g., between about 1.6 and 3.9 Debye, and include polyols, namely,diols and triols, e.g., ethylene glycol, poly ethylene glycol andglycerol; organic acids, including monoprotic and polyprotic acids,e.g., diprotic acids such as adipic acid, pimelic acid and malonic acid.Preferred are diprotic acids of the formula HOOC(CH₂)_(n)COOH, wherein nis an integer of not less than 3.

Another aspect of the invention relates to a process for controllingheavy metal emission in SO₂-containing flue gas, comprising passing astream of the flue gas through a wet scrubber where it is brought intocontact with a liquid absorbent comprising an ionic liquid and anoxidizer, wherein at least one organic acid is present in the liquidabsorbent. The organic acid is preferably a polyprotic acid, e.g., ofthe formula HOOC(CH₂)_(n)COOH (n≧3).

The water content of the flue gas needs to be taken into account whenselecting the polar protic organic solvent. The polyprotic acidsidentified above, in particular of the formula HOOC(CH₂)_(n)COOH, areespecially advantageous when cold water vapors are present in the fluegas.

The concentration of the polar protic organic solvent is at least 5 wt%, and preferably at least 10 wt %, e.g. from 10 to 50%, of the totalweight of the liquid absorbent, and more preferably from about 30-40(w/w).

As already mentioned above, an important advantage gained by theaddition of the polar protic organic solvent to the sorbent in the wetscrubber is that the reduced affinity of the ionic liquid toward sulfurdioxide is achieved at a temperature lower than the normal workingtemperature that would be used in the absence of said additive.Therefore, the process of the invention is suitably carried out at atemperature in the range from 85 to 170° C., e.g., 90 to 170° C., andmore preferably from 100 to 170° C. and even more preferably from 100 to140° C. The weight ratio between the polar protic organic solvent andthe ionic liquid is preferably at least 1:25, e.g., from 1:25 to 5:1,more specifically at least 1:10, e.g., from 1:10 to 2:1. For example,when the ionic liquid used is bromide ionic liquid, then the weightratio is at least 1:20 (e.g., the polar protic organic solvent is addedin an amount which is at least about 5% of the weight of the ionicliquid), such that a working temperature of less than 135° can beemployed, e.g., from 100 to 135° C. When the ionic liquid is chlorideionic liquid, then the weight ratio is at least 1:10, e.g., at least 1:5(e.g., the polar protic organic solvent is added in an amount which isat least about 10% of the weight of the ionic liquid), such that aworking temperature of less than 170° C. can be employed, e.g., from 100to 170° C., preferably from 100 to 135° C.

The description that follows refers to FIG. 3, which illustrates a fluegas scrubbing apparatus (wet scrubber) which is suitable for selectivelyremoving heavy metals from the flue gas, in accordance with the processof the invention. The apparatus comprises a gas-liquid contactor (1),which is typically in the form of a vertically positioned absorbertower, through which a gaseous stream flows in an upward direction andcontacts with a countercurrent stream of descending liquid. For example,a spray or packed tower may be used as the gas-liquid contactor. Theflue gas (2), which typically comprises from about 100 to 4000 ppmsulfur dioxide (e.g., about 1000 ppm sulfur dioxide) and from about 0.1to 2 ppb mercury (e.g., about 1 ppb mercury), enters the spray towerthrough an inlet pipe connected to the perimeter of the tower. The fluegas is caused to flow in an upward direction through the tower using ablower (3) which may operate at throughput of about 0.1 to 2,000,000m³/hour. Optionally, the flue gas is passed through a heat exchanger (4)prior to being introduced into the tower, where its temperature isreduced to less than 200° C., e.g., about 25 to 180° C.

The gas-liquid contactor is equipped with a plurality of spray headers(not shown) mounted in the internal space of the tower. The liquidsorbent is driven from a tank (6) into the upper section of the towerand is dispersed in the internal section of the tower through the sprayheaders. The liquid flows downward due to the force of gravity, contactswith the flue gas and oxidizes the heavy metals (e.g. mercury) in theflue gas. The absorption liquid is collected at the bottom of the tower,and is circulated through the use of a pump (7) and a circulation line(8) back to the upper section of the tower.

The oxidizer (e.g., elemental iodine) is held in a tank (11), and isperiodically injected to the circulation line (8) through a pipecontrolled by a valve (10), in response to indications received from ananalyzer (9) which measures the concentration of the oxidizer in thecirculating liquid.

Having been subjected to the scrubbing operation in the absorber tower,the upwardly flowing, essentially heavy metal-free flue gas exits thetower through a gas outlet opening (5) positioned in the upper sectionof the tower. A drop separator (not shown) may be mounted in uppersection of the tower, such that the gas permitted to escape from thetower is free of mist.

The liquid sorbent may be periodically treated to release therefrom themercury that was captured in the ionic liquid, thus refreshing the ionicliquid. It is noted that the mercury is in the form of mercury iodide(HgI₂), which exhibits high solubility in the ionic liquid. The mercuryis liberated from the ionic liquid by the addition of a reducing agentlike NaBH₄ or KHCO₂, which react with mercury iodide according to thefollowing reactions:

HgI₂ + NaBH₄ + H₂O→Hg⁰ + 2HI + NaBO₃ Room temprature KHCO₂ + HgI₂ +H₂O→Hg⁰ + 2HI + KHCO₃ T = 50 deg

The reduction of mercury iodide results in the formation a second liquidphase, consisting of elemental mercury. The two liquids phases (i.e.,the ionic liquid and Hg⁰) are easily separable from one another.

It should be noted that the addition of the polar protic organic solventidentified above to a ionic liquid may serve the useful purpose ofreducing the affinity of the ionic liquid toward acidic gases such assulfur dioxide in many industrial processes (in which there is a need toprevent the absorption of the acidic gas by the ionic liquid, or toliberate the acidic gas from the ionic liquid), allowing such industrialprocesses to be accomplished at a temperature lower than the normalworking temperature that would be used in the absence of said polarprotic organic solvent. Thus, the use of a polar protic organic solventfor reducing the affinity of ionic liquid toward acidic gases formsanother aspect of the invention.

An example of such process, which would benefit from the addition of apolar protic organic solvent to an ionic liquid, is the absorptionprocess of sulfur dioxide from a flue gas by means of ionic liquid. Insuch a process, the addition of the polar organic solvent would servethe useful purpose of liberating the SO₂ molecules absorbed and capturedby the ionic liquid, such that the ionic liquid could be easilyregenerated and recycled even at ambient conditions.

Accordingly, as already noted above, the process of the invention, whichis primarily directed to mercury removal from a SO₂-containing flue gas,may be advantageously preceded by an initial stage which is aimed atlowering the amount of sulfur dioxide in the flue gas. In this initialstage, a fluoride-containing liquid, e.g., a fluoride ionic liquid, canbe used as an absorbent, i.e., a cleansing liquid, for absorbing sulfurdioxide from flue gases, and the sulfur dioxide can be subsequentlydesorbed from the fluoride-containing liquid upon the addition of apolar solvent. Thus, the stage for SO₂ removal from the flue gasconsists of two successive steps: SO₂ absorption by afluoride-containing liquid, followed by SO₂ desorption and regenerationof the fluoride-containing liquid. The flue gas withdrawn from thistwo-step stage, having reduced level of SO₂, could then be directed tothe next stage of mercury absorption and oxidation as set out in detailabove.

The first step (SO₂ absorption) involves the absorption of sulfurdioxide from a gaseous mixture (in particular from flue gases formed bythe combustion of fossil fuels in electric power generating plants, orby engines operated by heavy fractions of oils, such as ship engines)through the use of a liquid absorbent that contains one or more fluoridesalts. The absorption temperature can be above 100° C., e.g., up to 160°C., e.g., from 110 to 140° C. The contact between the flue gases and theliquid absorbent is carried out in a suitable vertical structure knownin the art, namely, in a packed tower or a column, in which an upwardflow of gases to be purified and a countercurrent stream of a descendingliquid are allowed to mix, as described in more detail below.

The fluoride-containing liquid absorbent employed is a ionic liquid,i.e., a fluoride salt, or a mixture of fluoride salts, which are in aliquid state at a temperature below 150° C. Ionic salts which can beused according to the invention are composed of a fluoride-containinganion and substituted ammonium, imidazolium, pyridinium, pyrrolidiniumor phosphonium cations. Preferred are fluoride salts which contain anitrogen-containing cation, especially quaternary ammonium cation,namely, NR₁R₂R₃R₄ wherein each of R₁, R₂, R₃ and R₄ is independentlyC1-C10 alkyl group, preferably C1-C5 alkyl groups, or imidazolium,pyridinium, pyrrolidinium cation in which the nitrogen atom(s) arelinked to C1-C10 alkyl groups, such as 1-alkyl-3-methylimidazolium,1-alkylpyridinium and N-methyl-N-alkylpyrrolidinium, wherein the alkylgroup is preferably C2-C10 linear alkyl group, in particular C2-C5linear alkyl group.

Fluoride-containing ionic liquids are commercially available, sometimesin hydrated forms, and can also be synthesized by methods known in theart. For example, they can be prepared from the corresponding chloride,bromide or iodide salts, through the procedure of anion exchange thatwas described in the articles of Salman and Sasson [J. Org. Chem., 54,p. 4827-4829 (1989) and J. Org. Chem., 50, p. 879-882 (1985)]. Thehalide (non-fluoride) ionic liquids are generally prepared by reactionof the nitrogen-containing heteroaromatic ring with a suitable alkylhalide, as described above. The pure halide (X=Cl, Br or I) salt is thenmixed with a fluoride source in a polar protic solvent, for examplemethanol, in the presence of a small amount of water, such that thechloride, bromide or iodide counter ion is replaced by fluoride. To thisend, a reaction vessel is charged with the polar protic solvent, waterand the halide (X=Cl, Br or I) ionic liquid to form a solution. Theweight ratio methanol:ionic liquid is preferably between 3:1 and 3:2.The amount of water present in the reaction vessel is preferably lessthan 5% by mole, relative to the fluoride salt, e.g., between 2-4 mole%. The reaction mixture is stirred at room temperature (20-25° C.) toform a solution, followed by the addition of the fluoride source, whichis preferably an alkali salt, such as potassium fluoride. The fluoridesalt is used in a considerable molar excess relative to the halide(X=Cl, Br or I) ionic liquid, about 1.3-1.7:1 molar ratio of fluoridesalt to halide ionic liquid. The reaction mixture is maintained understirring at room temperature for not more than 30 min, followed byfiltration. The filtrate which is collected is treated in the samemanner, namely, by the addition of a fluoride source followed byseparation of solids, whereby a fluoride-containing ionic liquid isrecovered. A complete procedure is illustrated in the Preparationsbelow. In this way, a fluoride-containing ionic liquid, which issuitable for use as absorbent in the process of the invention, isobtained. Preferred fluoride ionic liquids suitable for use in theprocess of the invention include 1-butyl-3-methylimidazole fluoride,abbreviated [BMIM][F], and tetra alkyl (e.g., butyl) ammonium fluoride.It should be understood, however, that if the chloride, bromide oriodide counter ion is not fully replaced by the fluoride through theprocedure set forth above, then a mixed ionic liquid, e.g., of theformula [BMIM][F]_(y)[X]_(1-y) may be recovered (X=Cl, Br, I). The useof such mixed halide ionic liquids is also within the scope of theinvention.

According to another embodiment of the invention, the absorbent materialused for SO₂ removal is a mixture comprising at least one halide(non-fluoride) ionic liquid, together with a fluoride salt, e.g., afluoride ionic liquid as set forth above, wherein the two (or more)ionic liquids present in the absorbent are at least partially, andpreferably essentially completely, mutually miscible, such that theabsorbent is homogeneous. For example, the absorbent may be in the forma binary mixture consisting of either a chloride, bromide or iodideionic liquid in combination with the parallel fluoride ionic liquid. Bythe term “parallel” is meant that the ionic liquids present in themixture have the same cation and differ from one another with respect tothe halide counter anion. The mixtures set forth above benefit from thestabilizing environment provided by the chemically less reactivechloride/bromide/iodide salts, without sacrificing the excellentabsorbent capacity offered by the fluoride ionic liquid. Theexperimental work reported below indicates that the addition of arelatively small amount of the fluoride ionic liquid into the parallelhalide (non-fluoride) ionic liquid affords an absorbent having a highSO₂ absorption capacity, which can be easily regenerated and reused inrepeated absorbtion/desorption sequences. The molar ratio between thefluoride and the parallel halide ionic liquid in the absorbent may bebetween 1:99 and 99:1. A preferred mixture of halide ionic liquidssuitable for use in the invention consists of a first salt, which is[BMIMF] and a second salt, which is selected from the group of [BMIMCl],[BMIMBr] and [BMIMI].

Alternatively, anhydrous solution of a fluoride salt, such as alkalifluoride, e.g., KF, in halide ionic liquid is used as the liquidabsorbent.

The second step (SO₂-desorption) involves the addition of a polar-proticsolvent or a polar-aprotic solvent, or a mixture of such solvents, intothe SO₂-containing absorbent, in order to affect the desorption of thesulfur dioxide. Solvents which are suitable for use according to theinvention have polarity of not less than about 1.6 Debye (at 25° C.),e.g., between 1.6 and 3.9 debye, and include water, alkanols, ketones(e.g. acetone), organic acids (e.g., acetic acid), or mixtures thereof.The use of one or more C1-C5 alkanols, such as methanol, ethanol,isopropanol and butanol is preferred since these solvents are effectivein releasing the SO₂ molecule from the absorbent and are also easilyseparable from the absorbent due to their relatively low boiling point,by means employing conventional methods, e.g., distillation. In general,in order to achieve an essentially complete liberation of the SO₂ fromthe absorbent it is preferred that the ratio between the polar organicsolvent and the liquid absorbent be not less than 5% w/w, preferablybetween 40 to 200%. In the presence of the polar solvent, the desorptionof the sulfur dioxide can be accomplished at a temperature in the rangeof 5 to 160° C. and even at a lower temperature, most conveniently at atemperature lower than 100° C., e.g., between 20° C. and 60° C.,optionally under reduced pressure.

The absorption/desorption cycle outlined hereinabove can be tracked byanalyzing of Infrared (IR) spectrum of the absorbent. The characteristicIR stretching frequencies of the SO₂ molecule are at wavelengths ofabout 1151 cm⁻¹ and 1361 cm⁻¹. In the presence of the absorbentmaterial, these characteristic peaks shift slightly to lowerfrequencies. The exact positions of the absorption peaks attributed tothe presence of SO₂ in the IR spectrum of the SO₂-containing absorbentdepend on the composition of the absorbent. For example, as shown inFIG. 20, if SO₂ is absorbed in a mixture consisting of [BMIMBr] and 10%w/w of [BMIMF], then the IR spectrum exhibits the SO₂ stretching bandsat about 1125 cm⁻¹ and 1290 cm⁻¹. However, the IR absorption peaksattributed to the SO₂ molecule vanish upon the addition of the polarorganic solvent, such that the IR spectrum can serve as a useful toolfor monitoring the progress of the process of the invention.

Thus, in one embodiment of the invention, a process is provided, whereinthe SO₂-containing flue gas stream is treated before it is passedthrough the wet scrubber for removal of heavy metals, said treatmentcomprises contacting said SO₂-containing flue gas with afluoride-containing liquid absorbent comprising one or more fluoridesalts, whereby SO₂ molecules are captured by said fluoride-containingliquid absorbent and separated from the gaseous stream such that atreated flue gas stream having reduced SO₂ content is obtained, adding apolar solvent to said fluoride-containing liquid absorbent to remove thesulfur dioxide captured, regenerating and recycling thefluoride-containing liquid absorbent and directing the treated flue gasstream having reduced SO₂ content to the wet scrubber in which theremoval of the heavy metal is accomplished.

It should be understood, however, that the removal of SO₂ from fluegases as set out above should not necessarily follow with a step ofmetal (e.g., mercury) removal. Thus, the invention also provides aprocess for removing sulfur dioxide from gaseous mixtures, comprisingcontacting said gaseous mixture with a liquid absorbent comprising oneor more fluoride salts, adding a polar solvent to said absorbent toremove the sulfur dioxide from said absorbent and regenerating theabsorbent. Suitable fluoride salts, suitable polar solvents used asadditives and other process conditions are as described in detail aboveand below.

The description that follows refers to FIG. 14, which illustrates a fluegas scrubbing apparatus which is suitable for removing sulfur dioxidefrom the flue gases, in accordance with the process of the invention.The apparatus comprises a gas-liquid contactor (1), which is typicallyin the form of a vertically positioned absorber tower, through which agaseous stream flows in an upward direction and contacts with acountercurrent of descending liquid. For example, a spray tower may beused. The flue gas (2) enters the spray tower through an inlet pipeconnected to the perimeter of the tower. The flue gas is caused to flowin an upward direction through the tower using a blower (3) which mayoperate at throughput of about 0.1 to 2,000,000 m³/hr. Prior to beingintroduced into the tower, the flue gas is passed through a heatexchanger (4) where its temperature is reduced to less than 150° C.,e.g., about 60 to 150° C.

The tower is equipped with a plurality of spray headers (not shown)mounted in the internal space of the tower. The cleansing liquid isdelivered from a tank (6) into the upper section of the tower using apump (7) and is dispersed in the internal section of the tower throughthe spray headers. The liquid flows downward due to the force ofgravity, contacts with the flue gas and absorbs the sulfur dioxide.Having been subjected to the scrubbing operation in the absorber tower,the upwardly flowing, essentially SO₂-free flue gas is released to theatmosphere through a gas outlet opening (5) positioned in the uppersection of the absorber tower, or directed to the next stage of mercuryremoval. The cleansing liquid is collected at the bottom of the tower,and is recycled through the use of a pump (8) and a pipe (9) back to theupper section of the tower.

The separation of the sulfur dioxide from the cleansing liquid isaccomplished as follows. The SO₂-containing absorbent is driven througha pipe (10) to a stripping tower (11). The polar solvent, which is heldin tank (12), is charged into the stripping tower (11), and isthoroughly mixed with the SO₂-containing cleansing liquid in thestripping tower. The sulfur dioxide is removed under vacuum at arelatively low temperature (e.g., between 5 and 90° C.). The liberatedsulfur dioxide gas which exits the stripping tower via an outlet opening(13) is passed through a conduit (14) and is partially or totallycondensed in a condenser (15) and is finally recovered in a concentratedform suitable for further use, e.g., for the manufacture of sulfuricacid. Solvent traces collected in the condenser (15) are returned to thestripping tower (11) via conduit (16).

The polar solvent and the liquid absorbent can be separated from oneanother using conventional methods, e.g., through a distillationprocess. After the release of sulfur dioxide, the liquid mixture, whichconsists essentially of the fluoride-containing absorbent and the polarsolvent, is discharged from the bottom of the stripping tower (11) anddriven to a distillation unit (17). A schematic description of adistillation unit consisting of a distillation column and auxiliaryequipment is shown in FIG. 15. A distillation column (17) is chargedwith the liquid mixture (the liquid enters the column near its center).The distillation column (17) is equipped with a plurality of evenlyspaced plates (19), which are mounted in the column to define astripping section and rectifying section, below and above the feedplate, respectively. The liquid mixture flows down to the bottom of thecolumn, where a definite liquid level (20) is maintained. The liquid isremoved from the bottom of column (17) and fed into a boiler (21). Thevapor (22) generated by the boiler is fed at the low section of column(17). The vapor flows through the column (17), removed from the uppersection thereof, and condensed in a condenser (18). A liquid stream(reflux) is delivered (23) to the column to form a flow of a descendingliquid in the column. The overhead and bottom products thus recoveredare the polar solvent and the absorbent liquid, respectively. Returningback to FIG. 14, the polar solvent and the absorbent liquid arecollected in tanks (12) and (6), respectively, and can be reused.

In the Figures:

FIG. 1 provides a schematic illustration of the experimental set up usedfor measuring SO₂ absorption.

FIG. 2 provides a schematic illustration of the experimental set up usedfor measuring Hg absorption.

FIG. 3 illustrates a flue gas scrubbing apparatus (wet scrubber) whichis suitable for selectively removing heavy metals from the flue gas.

FIG. 4 is a graph showing the absorption of SO₂ in an ionic liquid inthe presence of different organic solvents.

FIG. 5 is a graph showing the absorption of SO₂ in an ionic liquid inthe presence of glycerol and ethylene glycol.

FIG. 6 is a graph comparing between the absorption of SO₂ and Hg in anionic liquid in the presence of adipic acid.

FIG. 7 is a graph showing the absorption of SO₂ in two different ionicliquid in the presence of ethylene glycol as additive.

FIG. 8 is a graph demonstrating the temperature dependence of SO₂absorption in different ionic liquids.

FIG. 9 is a graph showing how the absorption of SO₂ in an ionic liquiddepends on the anion of the liquid.

FIG. 10 is a graph demonstrating the effect of iodine addition to theionic liquid on the absorption of SO₂.

FIG. 11 is an illustration of the experimental set-up used for measuringthe effect of iodine addition on SO₂ absorption.

FIG. 12 is a graph showing a calibration curve, on the basis of whichiodine concentration can be measured.

FIGS. 13 and 13B demonstrate the favorable effect of the addition ofdiprotic acids to the ionic liquid when the gas stream to be absorbedcomprises water vapors.

FIG. 14 illustrates a scrubbing apparatus suitable for SO₂ removal.

FIG. 15 is a schematic diagram of a continuous distillation column whichcan be used in the regeneration of the fluoride-containing absorbent.

FIG. 16 is a graph showing the SO₂-absorption capacity of halide ionicliquids plotted versus time.

FIG. 17 is a graph illustrating the temperature dependence of theSO₂-absorption capacity of some halide ionic liquids.

FIG. 18 is a graph illustrating the SO₂-absorption capacity of somebinary mixtures consisting of fluoride and bromide ionic liquids.

FIG. 19 is a graph depicting the SO₂-absorption capacity of regeneratedbinary mixtures of halide ionic liquids.

FIG. 20 shows the IR spectra of an fluoride-containing absorbent {amixture of [BMIM][Br] and [BMIM][F] } under the following conditions:prior to SO₂ absorption (i), following SO₂ absorption (ii) and after SO₂desorption by the addition of methanol (iii).

FIG. 21 shows the IR spectra of a fluoride-containing absorbent {amixture of [BMIM][Cl] and [BMIM][F]} under the following conditions:prior to SO₂ absorption (i), following SO₂ absorption (ii) and after SO₂desorption induced by the addition of isopropanol (iii).

EXAMPLES Materials

Sulfur dioxide 5% (w/w) in nitrogen was purchased from Maxima gassupplier, Israel.

Mercury was purchased from Sigma Aldrich, Israel.

Tetra butyl ammonium fluoride (TBAF), butyl chloride, butyl bromide andbutyl iodide were purchased from Sigma Aldrich. 1-methylimidazole waspurchased from Merck.

Methyltrioctylammonium bromide, methyltrioctylammonium iodide, DMSO,1-methylnaphthalene, ethylene glycol, glycerol, adipic acid, methanol,isopropanol and potassium fluoride were purchased from Sigma Aldrich.

Aliquat 336 (methyltrioctylammonium chloride) was purchased from HollandMoran, Israel.

Measurements

SO₂ concentration was determined using a 3SF CiTiceL analyzer from CityTechnology Ltd, gas analyzer manufactured by Emproco ltd Israel.

Mercury concentration was determined using HG-MONITOR 3000 by SeefelderMesstechnik, Germany.

UV-Vis spectra were obtained using Cary 100 Bio spectrophotometer byVarian.

ATR-FTIR studies were conducted using Attenuated Total ReflectanceFourier Transform Infrared (ATR FTIR)—an Alpha model spectrometer,equipped with a single reflection diamond ATR sampling module,manufactured by Bruker (Ettlingen, Germany).

Elemental analysis was conducted using Perkin Elmer 2400 Analyzer.

SO₂ Absorption Measurement Setup

The experimental setup used for the measurement of SO₂ absorption in thefollowing examples is shown schematically in FIG. 1. A mixture of SO₂and air was made to flow through mass flow controller 102 into a gastrap 103 loaded with 10 grams of absorbing medium. The SO₂ source 101was a commercial 5% SO₂ gas cylinder (in N₂). The flow rates for the airand the 5% SO₂ gases were 1 L/minute and 8 ml/minute, respectively. Theconcentration of SO₂ in the air leaving the gas trap through conduit 104was analyzed by analyzer 105.

The SO₂ absorption yield is calculated as follows. The initial SO₂concentration was measured by using a bypass 106, through which thegases flow directly into the analyzer, thus determining the SO₂concentration at time zero. Subsequently the absorber trap wasconnected, and the SO₂ gas concentration in the outlet of the trap wasmeasured. The absorption yield is calculated by the following formula:

% Absorption=[SO_(2(time zero))−SO_(2(measured))]/SO_(2(time zero)).

Mercury Absorption Measurement Setup

FIG. 2 is a schematic illustration of the experimental setup used forthe measurement of mercury absorption. Air is caused to flow into amercury source 201 at a flow rate of about 2 liter/minute. The outgoingair stream, comprising mercury vapor, is directed through a conduit intoa gas trap 202 loaded with 10.0 grams of absorbing medium. The contacttime of the air in the absorbing medium is 0.2 seconds. Air leaving thegas trap through conduit 204 is analyzed by analyzer 205 for thepresence of mercury.

The mercury absorption yield is calculated as follows. The initialmercury concentration was measured by using a bypass 203, through whichthe mercury-comprising air flows into the analyzer, thus determining themercury concentration at time zero. Subsequently the absorber trap wasconnected, and the mercury concentration in the outlet of the trap wasmeasured. The absorption yield is calculated by the following formula:

% Absorption=[Hg_((time zero))−Hg_((measured))]/Hg_((time zero)).

Preparations 1-3 Preparation of 1-butyl-3-methylimidazolium halide(chloride, bromide and iodide)

1-bromobutane (110 mmol, 15.07 grams) and 1-methylimidazole (100 mmol,8.21 grams) were added to a 250 mL flask. The reaction mixture wasstirred for 48 hours at 80° C. The resulting ionic liquid was thencooled, washed with ether (3×25 mL) to remove unreacted startingmaterials, and the product was dried under vacuum at 80° C. for 4 hoursto afford 1-butyl-3-methylimidazolium bromide [BMIMBr] in a yield of 93%with 96% purity. The procedure was repeated using 1-chlorobutane and1-iodobutane to give the corresponding chloride and iodide salts.

The complex of [BMIMX] [I₂] was formed by addition of solid iodine intothe ionic liquid (for example [BMIMBr]), at room temperature. Thecomplex of [BMIMX] [Br₂] was formed by careful addition of liquidbromine to the ionic liquid under room temperature.

Preparation 4 Preparation of 1-butyl-3-methylimidazoliumtetrafluoroborate

1-butyl-3-methylimidazolium chloride (30 g) was dissolved in 35 ml ofwater in an Erlenmeyer (125 ml). NaBF₄ (20 gram) was gradually addedunder mixing during a period of 10 to min. The temperature dropped downto about 14° C. The reaction mixture was allowed to return to roomtemperature, following which 30 ml of dichloromethane were added. Thereaction mixture was separated into organic and aqueous phases using aseparatory funnel. The organic phase was removed, and the aqueous phasewas extracted again with 20 ml of dichloromethane. Following phaseseparation, the organic phase was removed and the two organic phaseswere combined together in a separatory funnel and shaken with 10 gram ofNaBF₄ dissolved in 20 ml of water. The sample was dried by using amixture of 1 g of Na₂SO₄ and 3 g of MgSO₄, and the solids were filteredout using of a Buchner funnel.

Preparation 5 Preparation of 1-butyl-3-methylimidazolium fluoride

10.3 grams of [BMIMCl] prepared as described above were added to amixture of 28 mL methanol and 0.06 mL water. After complete dissolutionof the [BMIMCl], 5.0 grams of dry potassium fluoride (KF) were added,and the reaction mixture was stirred at 22° C. for 20 minutes. Themixture was filtered. 4.4 grams of KF was added to the filtrate, and themixing continued for additional 20 minutes. After filtration, themethanol was evaporated under vacuum at room temperature, and themixture was then filtered and centrifuged. The resulting yellowishliquid was determined by elementary analysis to be the titled compound[BMIMF] in 94% yield.

The next set of examples (Examples 1 to 5) demonstrates how to carry outthe selective absorption of mercury by an ionic liquid, when the“competitor”, sulfur dioxide, is also present in the gaseous stream tobe treated.

Example 1 The Absorption of Mercury and SO₂ by an Ionic Liquid in thePresence of Different Organic Solvents

The effect of the addition of different organic solvents to anabsorption medium comprising ionic liquid and an oxidizer was tested asfollows. The absorption of SO₂ in [BMIMBr][I₂] (80 mg of iodine in 12grams of ionic liquid), to which ethylene glycol was added, was measuredat different concentrations of the ethylene glycol (0, 5, 10, 15, 20, 25and 30% w/w) using the experimental setup described above. For thepurpose of comparison, similar experiments were carried out under thesame conditions using either DMSO or 1-methylnaphthalene at differentconcentrations instead of ethylene glycol.

The results are graphically presented in FIG. 4, where the abscissaindicates the concentration of the additive (indicated as molar percentrelative to the ionic liquid) and the ordinate indicates the degree SO₂absorption by the absorption medium. It may be appreciated that theaddition of a polar protic organic solvent such as ethylene glycoldecreases the SO₂ absorption. No change in the SO₂ absorption wasobserved following the addition of aprotic polar solvent such as DMSO,or a nonpolar solvent like 1-methylnaphthalene.

Having determined that the addition of a polar protic organic solventreduces the absorbtion of sulfur dioxide in the ionic liquid, the effectof the addition of such a solvent on the absorbtion of mercury has beeninvestigated. For this purpose, the absorption of mercury and SO₂ at 70°C. by a liquid absorbent comprising [BMIMBr][I₂] (80 mg of iodine in 12grams of ionic liquid) and different concentrations of polar proticorganic solvents (ethylene glycol, glycerol and adipic acid) wasmeasured seperately, using the two experimental setups descibed above.

The absorption of SO₂ as a function of the molar percent of the polarprotic organic solvent relative to the ionic liquid is depicted in FIG.5 for glycerol (rhombous) and ethylene glycol (squares) and in FIG. 6for adipic acid. While the addition of polar protic organic solvents hadno effect on the absorption of mercury, which remained over 95% (i.e.,with adipic acid 97%—see FIG. 6) with ethylene glycol 98% and withglycerol 97%—not shown), the SO₂ absorption decreased gradually with theincreaed addition of either glycerol, ethylene glycol or adipic acid.

Example 2 The Change in SO₂ Absorption Exhibited by Different IonicLiquids in Response to the Addition of a Polar Protic Organic Solvent

The maximal absorption of SO₂ at 70° C. in either [BMIMBr][I₂] or[BMIMCl][I₂], (80 mg iodine in 12 grams of ionic liquid), to whichethylene glycol was added at different amounts, was measured using theexperimental setup descibed above.

FIG. 7 illustrates the absorption of SO₂ as a function of the ethyleneglycol amount in the liquid absorbent. A shaper drop in the SO₂absorption in reponse to the addition of ethylene glycol is observed for[BMIMBr][I₂] in comparison to [BMIMCl][I₂], indicating that the formeris more preferred for use in the selective absorption of mercury overSO₂.

Example 3 The Combined Effect of Temperature and Addition of a PolarProtic Organic Solvent on the SO₂ Absorption Demonstrated by DifferentIonic Liquids

The maximum absorption of SO₂ by different ionic liquids (i.e., TBAF,[BMIMCl], [BMIMBr], [BMIMI] and [BMIMBF₄]) was measured at differenttemperatures in the range from 25 to 120° C., using the experimentalset-up described above.

The temperature dependence of the absorption of SO₂ by the ionic liquidsis depicted in FIG. 8 (for TBAF, [BMIMCl], [BMIMBr] and [BMIMI]) andFIG. 9 (for [BMIMI] and [BMIMBF₄]).

As shown in the graph of FIG. 8, the absorption of SO₂ by TBAF is notinfluenced by temperature changes, rendering it unsuitable for use forthe selective absorption of mercury over SO₂. However, the affinity ofthe chloride-, bromide- and iodide-containing ionic liquid toward SO₂decreases with increasing temperature. Furthermore, the larger thehalide, the sharper is the decrease in the affinity in response totemperature increase.

Turning now to the graph of FIG. 9, it is noted that [BMIMBF₄] exhibitsa particularly low affinity towards SO₂: the absorption of SO₂ by[BMImBF₄] drops to 0% at a temperature as low as 90° C. This isconsistent with observation made above with respect to the inversecorrelation between the size of the anion and its affinity toward SO₂(BF₄ ⁻ is a relatively large anion). The results shown in FIG. 9 suggestthat a mixture of [BMImBF₄] or [BMIMPF₆] and [BMIMX][I₂], wherein X ishalide, preferably bromide or iodide, is useful in the selective removalof mercury from SO₂-containing flue gas at a relatively low workingtemperature, e.g., around 90-100° C., with [BMIMBF₄] providing a bulkwith a remarkably low affinity towards sulfur dioxide and [BMIMX][I₂]acting as the active agent permitting the oxidation of the mercury.

It should be noted that FIG. 9 provides the results of an experiment inwhich the ionic liquid contained 3-5% water.

Example 4 The Effect of Oxidizer Concentration on SO₂ Absorption

The effect of the concentration of the oxidzer (elemntal iodine) presentin the liquid adsorbent, on the absorption of sulfur dioxide was testedusing the experimental arrangment shown in FIG. 11.

A three-necked flask (401) was loaded with 20 mL of an aqoueous solutioncomprising sodium sulfite (Na₂SO₃) and water in a weight ratio of 1:2. 5mL of sulfuric acid (H₂SO₄) was gradually added to the flask through adropping funnel (402), allowing the formation of sulfur dioxide by thefollowing reaction:

Na₂SO_(3(aq))+H₂SO_(4(I))→Na₂SO_(4(aq))+SO_(2(g))+H₂O_((l))  (II)

Air comprising the sulfur dioxide gas thus formed was allowed to flowthrough a water trap (403) to a gas trap (404) loaded with 4 ml, of anabsorption medium comprising [BMIM][Br] and [I₂] in various ratios(between 1:0 and 1:2), where some of the SO₂ was absorbed. The airleaving the gas trap through conduit (405) entered vessel (406),containing an aqueous calcium carbonate solution, which absorbed thesulfur dioxide that was not previously absorbed by the absorption mediumin the gas trap, following which the air was released into theatmosphere.

The sulfur dioxide absorption was determined by measuring the weight ofthe absorption medium before and after each experiment; the weightdifference is attributed to sulfur dioxide absorbed by the ionic liquid.The absorption yield was calculated as follows:

% Absorption=[Ilw_((measured))−Ilw_((time zero))]/Ilw_((time zero))

Ilw=Ionic liquid weight

The results are depicted in FIG. 10, which shows the sulfur dioxideabsorption percent as a function of the iodine concentration in theabsorption medium. It may be appreciated that as the iodineconcentration increases, the sulfur dioxide absorption by the liquiddecreases, dropping from 70% for iodine-free liquid to approximately 0%for a liquid comprising the ionic liquid and iodine at a ratio of 1:2 atroom temperature.

Example 5 The Effect of Oxidizer Concentration on SO₂ Absorption

The experimental set-up illustrated in FIG. 1 was operated at roomtemperature. The sorbent tested was [BMIMBr][I₂], with equal molar ratiobetween the ionic liquid [BMIMBr] and the iodine. The concentration ofsulfur dioxide in the air stream that passed through the gas trap 103was 1000 ppm. Under these conditions, no absorption of sulfur dioxidewas noted.

Example 6 Tracking the Iodine Concentration Using UV-VisSpectrophotometer

A set of experiments was carried out in order to determine the stabilityof iodine in the ionic liquid, in the case where the gas stream that isbrought into contact with the ionic liquid contains also water. For thispurpose, the experimental set-up described in FIG. 1 was modified inorder to allow the introduction of cold water vapors into the gaseousstream. A water trap at room temperature was placed between the massflow controller 102 and the gas trap 103 loaded with the liquidabsorbent. Also, the gas trap 103 was coupled to a heater.

The variables that were investigated include the working temperaturesand the polar protic organic solvent added to the ionic liquid. In eachexperiment, the experimental set-up was operated for three hours at aselected temperature, testing the efficacy of the polar protic organicsolvent in preventing the reduction of iodine at that temperature. Theconcentration of iodine in the ionic liquid was measured using UV-Visspectrophotometry at a wavelength of 281 nm based on calibration curves(such as the one shown in FIG. 12 for [BMIMBr][I₂]. The iodineconcentration was measured at the beginning of each experiment and thenat intervals of 1.5 hours.

The favorable effect of the presence of diprotic acids such as adipicacid on the absorption temperature, namely, the working temperatureneeded to prevent SO₂ absorption and reduction of iodine to iodide inthe presence of water vapors in the gas to be treated, is illustrated bythe data in the following table:

Absorption temperature Ionic liquid Without With additive Amount ofadditive and oxidizer additive (adipic acid) needed (W/W) [BMIMBr][I₂]140° C. 100-135° C. 100-5%  [BMIMCl][I₂] 180° C. 100-170° C. 100-10%

As shown by the graphs of FIGS. 13A and 13B, absent the additive, theabsorption temperature should be not less than 140° C. (see 13A) inorder to maintain the iodine in its elemental form, which is necessaryfor the removal of mercury. The addition of diprotic acids permits theprocess to run at a lower temperature, e.g., 110° C., while preventingthe reduction of iodine by sulfur dioxide (see 13A and 13B). Incontrast, in the absence of a diprotic acid, the iodine is reduced bysulfur dioxide and its concentration following the three hours testdropped to zero.

The next set of examples (Examples 7 to 10) demonstrates how sulfurdioxide can be separated from a gaseous mixture by the use offluoride-containing ionic liquid, and how that ionic liquid can later berecovered. The withdrawn gaseous stream (with reduced SO₂ level) canthen be treated to permit mercury removal under favorable conditions.

Example 7 SO₂ Absorption b Tetra but 1 Ammonium Fluoride TBAF IonicLiquid

The absorption of SO₂ by tetra butyl ammonium fluoride (TBAF) at roomtemperature was measured using the experimental set-up described above.The experiment was allowed to continue for ten minutes, during which theSO₂ absorption was measured periodically. For the purpose of comparison,similar experiments were carried out under the same conditions using twonon-fluoride ionic liquids: [BMIMCl] and [BMIMI].

The results are presented in FIG. 16 which shows the SO₂ absorption (aspercent relative to the initial SO₂ concentration) against time (inseconds), for each of the three ionic liquids (the curves indicated byrhombuses, squares and triangles correspond to TBAF, [BMIMCl] and[BMIMI], respectively). It may be appreciated that TBAF reaches anabsorption value of over 98% compared with 93% for [BMIMCl] and 84% for[BMIMI].

The maximum absorption of SO₂ by TBAF was also measured at differenttemperatures between 25 and 100° C., using the experimental set-updescribed above. For comparison, the experiment was carried out underthe same conditions using three other ionic liquids: [BMIMCl], [BMIMI]and [BMIMBr].

FIG. 17 shows the temperature dependence of the SO₂ absorption by halideionic liquids for the four halide ionic liquids. The absorption of SO₂by the TBAF ionic liquid is approximately 100% throughout the entiretemperature range (25-120° C.), whereas the absorption by the otherionic liquids was reduced considerably upon temperature elevation.

Example 8 SO₂ Absorption by a Binary Mixture Consisting of [BMIMF] and[BMIMBr] Ionic Liquids

The maximal absorptions of SO₂ at 50° C. by means of various binarymixtures consisting of [BMIMF] and [BMIMBr] were measured using theexperimental set-up described above. The experiments were carried outusing four mixtures having different compositions, namely, the variableinvestigated in the experiment was the weight ratio between [BMIMF] and[BMIMBr] in the binary mixture (0% [BMIMF], 5% [BMIMF], 10% [BMIMF] and13% [BMIMF]; wt %).

The results are graphically presented in FIG. 18, where the abscissaindicates the concentration of the [BMIMF] component in the mixture andthe ordinate indicates the SO₂ absorption by the mixture. It may beappreciated that the addition of [BMIMF] to [BMIMBr] causes asignificant increase in SO₂ absorption. The absorption of SO₂ by anionic liquid mixture having a [BMIMF]:[BMIMBr] ratio of 1:20 reaches91.6%, whereas the absorption by 100% [BMIMBr] reaches only 67.1%.

Example 9 SO₂ Desorption from Various Fluoride Absorbents andRegeneration of the Absorbents

The absorption of SO₂ by 10 grams of TBAF ionic liquid was allowed tocontinue for 25 minutes, using the experimental set-up of FIG. 1. Duringthis period of time, the SO₂ absorption was measured periodically.Subsequently, 20 mL of methanol was added to the TBAF.

The solution obtained was mixed at room temperature in an open vesselfor 20 minutes. Fourier transform infrared spectroscopy (FTIR) analysisconfirms the release of the SO₂ from the solution. The absorption of SO₂by TBAF gave rise to the formation of new peaks (e.g. at 1087 cm⁻¹ and1216 cm⁻¹) in the spectrum of the ionic liquid, attributed to the SO₂.The addition of methanol caused these peaks to disappear, indicating thedesorption of SO₂ from the ionic liquid. It should be noted that therelease of the SO₂ from the absorbent was accomplished through theaddition of the polar solvent alone, without increasing the temperatureof the absorbent.

In order to evaluate the regeneration efficacy of the absorbent, theabove experiment was repeated under the same conditions using a mixtureof 1:9 [BMIMF]:[BMIMCl] as the ionic liquid absorbent. After the SO₂ wasdesorbed through the addition of methanol, the methanol was evaporatedunder vacuum at 45° C. and the regenerated absorbent was used forconsecutive cycles of absorption/desorption. A cycle consisting ofabsorption, SO₂ release by means of methanol addition and the removal ofthe methanol was repeated three times using the same ionic liquidabsorbent.

FIG. 19 depicts the absorption of SO₂ (percent relative to the initialSO₂ concentration) as a function of time (seconds) for each of the threeabsorption/desorption cycles. As shown, the efficiency of SO₂ absorptionremained constant throughout the three cycles, reaching over 90%absorption, indicating that the absorbent has been effectivelyregenerated.

Example 10 SO₂ Desorption from the Absorbent

The absorption/desorption procedure outlined in the previous example wasrepeated, using a mixture of [BMIMF] and [BMIMCl] (1:9) as the absorbentmaterial. The polar solvent added to the SO₂-containing absorbent wasisopropanol, with volumetric ratio of 1.5:1 in favor of the latter. Asshown in the IR spectra of FIG. 21, peaks assigned to the SO₂ moleculein the SO₂-containing absorbent are at 1279 and 1120 cm⁻¹. Thedisappearance of the aforementioned IR bands after the addition of thepolar solvent, isopropanol, indicates the release of the sulfur dioxide.

1) A process for controlling heavy metal emission in SO2-containing flue gas, comprising passing a stream of the flue gas through a wet scrubber where it is brought into contact with a liquid absorbent comprising an ionic liquid, an oxidizer and polar protic organic solvent, wherein the amount of said organic solvent is adjusted such that the S0₂ absorption is minimized while operating said wet scrubber at a working temperature lower than the normal working temperature that would be used in the absence of said solvent. 2) A process according to claim 1, wherein the liquid absorbent comprises bromide ionic liquid, with the weight ratio between the polar protic organic solvent and said ionic liquid being at least 1:20 and wherein the working temperature is from 100 to 135° C. 3) A process according to claim 1, wherein the liquid absorbent comprises chloride ionic liquid, with the weight ratio between the polar protic organic solvent and said ionic liquid being at least 1:10 and wherein the working temperature is from 100 to 170° C. 4) A process according to claim 2, wherein the ionic liquid comprises 1-butyl-3-methylimidazolium bromide or 1-butyl-3-methylimidazolium chloride, respectively, and the oxidizer is iodine. 5) A process according to claim 4, wherein the ionic liquid further comprises at least one additional 1-butyl-3-methylimidazolium salt selected from the group consisting the BF₄ ⁻ and PF₆ ⁻ salts. 6) A process according to claim 1, wherein the polar protic organic solvent comprises a diprotic acid. 7) A process according to claim 6, wherein the diprotic acid has the formula HOOC(CH₂)_(n)COOH, wherein n is an integer of not less than
 3. 8) A process according to claim 7, wherein the acid is adipic acid and/or pimelic acid. 9) A process according to claim 1, wherein the heavy metal is mercury. 10) A process according to claim 1, wherein the S0₂-containing flue gas stream is treated before it is passed through the wet scrubber for removal of heavy metals, said treatment comprises contacting said S0₂-containing flue gas with a fluoride-containing liquid absorbent comprising one or more fluoride salts, whereby S0₂ molecules are captured by said fluoride-containing liquid absorbent and separated from the gaseous stream such that a treated flue gas stream having reduced S0₂ content is obtained, adding a polar solvent to said fluoride-containing liquid absorbent to remove the sulfur dioxide captured, regenerating and recycling the fluoride-containing liquid absorbent and directing the treated flue gas stream having reduced S0₂ content to the wet scrubber in which the removal of the heavy metal is accomplished. 11) The process of claim 10, wherein the fluoride-containing liquid absorbent comprises one or more fluoride ionic liquids. 12) The process of claim 10, wherein the fluoride-containing liquid absorbent is a mixture of halide ionic liquids, said mixture comprises, in addition to the fluoride ionic liquid, also at least one chloride, bromide or iodide ionic liquids which are miscible with the fluoride ionic liquid. 13) The process of claim 10, wherein the solvent added to the fluoride-containing liquid absorbent is a polar protic solvent. 14) The process of claim 13, wherein the polar protic solvent is selected from the group consisting of C1-C5 alkanols. 15) The process of claim 10, wherein the treatment comprises the absorption of the sulfur dioxide at a temperature from 100° C. to 160° C. 16) The process of claim 15, wherein the absorption temperature is from 110 to 140° C. 17) A process for removing sulfur dioxide from gaseous mixtures, comprising contacting said gaseous mixture with a liquid absorbent comprising one or more fluoride salts, adding a polar solvent to said absorbent to remove the sulfur dioxide from said absorbent and regenerating the absorbent. 18) The process of claim 17, wherein the absorbent comprises one or more fluoride ionic liquids. 19) The process of claim 18, wherein the absorbent is a mixture of halide ionic liquids, said mixture comprising at least one chloride, bromide or iodide ionic liquids which are miscible with the fluoride ionic liquid(s). 20) The process of claim 17, wherein the solvent added to the absorbent is a polar protic solvent. 21) The process of claim 20, wherein the polar protic solvent is selected from the group consisting of C1-C5 alkanols. 22) The process of claim 17, wherein the absorption of the sulfur dioxide by the absorbent is carried out at a temperature ranging from 100° C. to 160° C. 23) The process of claim 22, wherein the temperature is from 110 to 140° C. 24) A process for controlling metal emission in S0₂-containing flue gas, comprising passing a stream of the flue gas through a wet scrubber where it is brought into contact with a liquid absorbent comprising a ionic liquid and an oxidizer, wherein the molar ratio between the oxidizer and the ionic liquid is not less than 1:2, and wherein the absorption temperature is from 20° C. to 35° C. 25) A process for controlling heavy metal emission in S0₂-containing flue gas, comprising passing a stream of the flue gas through a wet scrubber where it is brought into contact with a liquid absorbent comprising an ionic liquid and an oxidizer, wherein at least one organic acid is present in the liquid absorbent. 